Warning: Title::newFromTitle: $text must be a string:
from
HTMLForm::tryAuthorizedSubmit/HTMLForm::trySubmit/SpecialGlobalRenameProgress::onSubmit/User::getCanonicalName/Title::newFromText

	function onSubmit( array $data ) {
		$name = User::getCanonicalName( $data['username'], 'usable' );
		            //\-> Title::newFromText

Looks like username can be unset (and yield null), or otherwise contain a non-string value.